1. Field of the Invention
The present invention relates to techniques for mounting an integrated circuit chip to a substrate in a multi-chip module (MCM), and more specifically to a structure having a heat spreader with intermediate bonding pads for connecting the chip to the MCM.
2. Description of the Related Art
Technical evolution in the field of electronics has resulted in a demand for faster and more compact systems. More advanced applications also necessitate greater numbers of components, which need to communicate with each other. For new systems to meet the requirement of quick access between different components, the length of the paths between different components of the system must be kept within certain limits. However, when the complexity of a system grows, the length of the paths between components also grows. In order not to exceed the maximum allowed distance between such components, these components have been built smaller and have been packed more densely.
Such constraints have given rise to the MCM. An MCM, as is known, comprises a package containing a plurality of interconnected integrated circuits (“ICs,” “dies,” or “chips”). The package of the MCM supports a lead frame having internally to the package a plurality of connector points (lead fingers) for coupling to the ICs, and having externally to the package a plurality of pins coupleable to a printed circuit board for example.
Internal to the MCM, the ICs are supported by a substrate. Typically, the substrate is insulative but contains conductive electrical traces to allow signals to be routed along the substrate between two or more of the ICs. Ultimately, the ICs in the MCM are electrically coupled by connecting the bond pads of the ICs to the traces, or by directly connecting the bond pads of the ICs together. In either case, such connection usually occurs through the use of bond wires, although “flip chip” technologies may also be used in which the ICs are overturned in the MCM package to bring IC bond pad bumps into contact with traces on the substrate. In any event, once the ICs are interconnected, and once connected to the lead frame of the package, the circuitry and the various connections (e.g., bond wires) are protected by filling the package with a potting material, and/or securing a lid to the package.
FIG. 1 illustrates a top-down view of the inside cavity of a package of a MCM 1. As shown, multiple ICs 2(a-c) are mounted to a surface 3a of substrate 3. Various substrates useable in MCMs are known in the art, such as low-temperature co-fired ceramics (LTCCs) which are made of one or more layers of ceramic material and are typically bonded to a metal material for support (not shown). As shown, each IC 2 includes bond pads 4 that allow the ICs to be electronically connected to other components of the MCM. The surface 3a of the substrate 3 also contains bond pads 6 which connect to traces 30 used to route the signals inside the MCM 1, only a few of which are shown for clarity. As noted above, bond wires 5 are one way of making the necessary connections, and such connections can either connect the IC bond pads 4 to the bond pads 6 of the traces 30 (5a), or directly connect two IC bond pads 4 without the need to use an intervening trace 30 (5b). Bond wires Sc can also be used to ultimately connect either a trace bond pad 6 or an IC bond pad 4 to the lead frame 7 inside the package, which as noted emerges outside the package as a pin.
ICs 2 generate heat when they operate, and this can cause particular problems in an MCM 1 because many ICs are packed densely together. As is known, excessive heat can compromise the operation of an IC, and in turn can compromise the performance of the MCM 1. One method of combating heat generation has been to provide a heat spreader 9 between an IC 2a that generates excessive heat and the substrate surface 3a, as shown in FIG. 2. Heat spreader 9 is a layer of a thermally conductive material, for example a metal such as copper. Heat spreader 9 spreads the heat generated by IC 2a over an extended area, thus facilitating heat dissipation and also mitigating problems which might be caused by isolated “hot-spots” on the IC 2, which can cause the IC to malfunction.
While addressing the problem of heat accumulation, however, the implementation of heat spreader 9 compromises the dependability of MCM 1 because the distance 6a between IC bond pads 4 and the trace bond pads 6 is substantially increased due to the spatial extent of the heat spreader 9. As a result, bond wires 5 connecting these two types of bond pads are also lengthened, which makes the MCM 1 prone to failure. Bond wires 5 that are appreciably long are also relatively heavy, and are thus prone to breaking, particularly when subject to mechanical vibrations. Also, the length and weight of the bond wires spanning over the heat spreader can cause such bond wires to “droop,” which puts them at risk for touching the conductive heat spreader 9 and shorting thereto. Thus, simply introducing a heat spreader 9 may not be a viable solution to the problem of heat accumulation in MCM 1, and improved solutions are needed. Moreover, while the spatial extent of the heat spreader 9 could be made smaller, this also reduces its heat dissipation properties, and hence does not provide a viable solution.
Size, temperature, complexity, or cost constraints generally hamper prior heat dissipation techniques utilized in MCMs. See, e.g., U.S. Pat. Nos. 6,455,930, 6,483,705, and International Patent Publication No. WO 01/43167, which are all incorporated herein by reference. It is therefore desirable to provide alternative techniques for mounting ICs on substrates in MCMs in a way that effectively dissipates the heat generated by the IC but is cost efficient, simple, easy to manufacture, and not prone to failure.